The present invention relates to a device for nebulizing a liquid, comprising a nebulization head having a capillary tube and a nozzle for ejecting a liquid, a liquid supply tank for supplying the nebulization head with liquid, linked to the nebulization head by a pipe, a vibrator for vibration driving the nebulization head, so that it ejects droplets of a liquid in a nebulization jet, and an exciter for applying an excitation signal to the vibrator.
A nebulization device of the above-mentioned type is described in International patent application publication WO 99/46126 (U.S. Pat. No. 6,460,980) in connection with the production of an inkjet print head.
However, various other applications of such a device may be made, particularly applications involving nebulization of liquids in the air, for purposes of humidifying or cooling the air, or for purifying diffusion, deodorizing, disinfecting products, perfumes, etc. as described for example in European published patent application EP 0 714 709 A or International patent application publication WO 00/78467. The present invention particularly relates to such applications.
FIG. 1 schematically represents the conventional structure of such a device. The device 10 comprises a nebulization head 20, an intermediate tank 21 containing a liquid 22 to be nebulized, a main tank 24 also containing liquid 22, a pipe 25 linking the tank 21 to the nebulization head 20, and a pipe 26 equipped with an electric pump 27, linking the tank 21 to the tank 24. The nebulization head 20, substantially horizontal, comprises a capillary tube 20-1 and an ejection nozzle 20-2 for ejecting the liquid. It generally has the form of a tubular needle, the inside diameter of which is less than one millimeter, the length of which is a few centimeters, the body of which forms the capillary tube 20-1, and the beveled, distal end of which forms the ejection nozzle 20-2. The nebulization head 20 is mechanically coupled to a vibrator, generally a resonating piezoelectric transducer 28, that is electrically powered by an AC signal Sv supplied by an excitation circuit EXCT. The excitation circuit EXCT is driven by a control circuit CNTCT that defines nebulization cycles whose duration varies according to the intended application.
In normal conditions of operation, the level of liquid in the intermediate tank 21 is at a height H1 from the longitudinal axis of the nebulization head 20. As the tank 21 is subjected to atmospheric pressure Patm, the liquid 22 present in the pipe 25, at the inlet to the nebulization head 20, is subjected to the atmospheric pressure to which an overpressure Ph1 is added, equal to the hydrostatic pressure imposed by the liquid column of height H1, where Ph1=ρ·g·H1, ρ being the density of the liquid and g the gravity.
When the excitation signal Sv is applied to the transducer 28, the nebulization head goes into resonance and an antinode appears at its end 20-2. Droplets 22-2 of liquid 22 are ejected in a direction substantially perpendicular to the plane of the beveled section of the nebulization head, forming a sort of mist of droplets or “nebulization jet.” During the nebulization, the nebulization head 20 is supplied with liquid by capillarity and by gravity (effect of the hydrostatic overpressure). An air circulator, such as a fan (not represented), for circulating the air can be provided to increase the range of the nebulization jet.
When idle, when the nebulization head is not vibration driven, the liquid 22 is retained in the nebulization head by capillarity, and the hydrostatic pressure is offset by the appearance of a convex meniscus 22-1 of liquid 22 at the end of the nebulization head, due to the surface tension forces acting on the liquid. Above a critical overpressure threshold Sh1, the meniscus 22-1 breaks and the liquid 22 flows through the nebulization head.
It results from the above that the level of the liquid in the intermediate tank 21 must be precisely controlled, so that the hydrostatic pressure is maintained below the threshold Sh1, which is generally very low and on the order of 50 to 150 Pa.
Thus, the optimum height H1 depends on the threshold Sh1 and the physico-chemical characteristics of the liquid, particularly the viscosity, the density, the surface tension forces, and on the inside diameter of the capillary tube of the nebulization head. This height is generally low, on the order of 5 to 15 mm with alcoholic or aqueous solutions and a nebulization head whose capillary tube has an inside diameter on the order of 0.6 mm. The height H1 is kept substantially constant by the circuit CNTCT, that monitors the level of liquid with a level detector 23 arranged in the tank 21, and activates the pump 27 from time to time.
According to a known improvement represented in FIG. 2, wherein no pumps need be used, the device comprises an intermediate tank 21′ linked to a main tank 24′ by a pipe 26′ according to the principle of communicating vessels. The tank 21′ is subjected to atmospheric pressure Patm, while the tank 24′ is hermetically closed and is subjected to a pressure P1 lower than atmospheric pressure, which prevents the tank 24′ from being entirely emptied into the tank 21′. When hydrostatic equilibrium is achieved, the level of the liquid 22 in the tank 24′ is above the level of the liquid in the tank 21′. When the level of liquid in the tank 21′ becomes lower than the high level of the pipe 26′, some air enters the tank 24′ and some liquid is transferred into the tank 21′. An auxiliary pipe 27′ can also be provided, so as to cause air to enter the upper part of the tank 24′. Thus, the height of liquid in the tank 21′ is automatically regulated.
Despite this improvement, the nebulization device just described has various other disadvantages. In particular, the device is very sensitive to changes in trim that make the liquid move in the intermediate tank, as well as to other phenomena producing similar effects, for example vibrations transmitted by the external environment. Such changes in trim or vibrations can cause droplets to flow at the end of the nebulization head. Indeed, as the height H1 is low, the movements of the surface of the liquid in the intermediate tank can lead to exceeding the critical threshold Sh1, thus breaking the meniscus 22-1 at the end of the nebulization head.
Another disadvantage of the conventional nebulization device is that its nebulization flow depends on the nature of the liquid that is nebulized. Flow rates that differ in a ratio ranging from 2 to 10 between liquids in aqueous solution and liquids in alcoholic solution, for the same nebulization head, can thus be observed. As an example, a nebulization head whose inside diameter is 0.6 mm and whose length is 27 mm, vibration driven at 200 kHz, enables flow rates to be obtained on the order of 1 to 3 grams per minute with liquids essentially made up of water (in which air treatment products are dissolved), but on the order of only 0.1 to 0.6 grams a minute with water-, ethyl alcohol- and dipropylene glycol-based solutions.